feat: Add mobile responsive layout, open room access, and admin room management

Mobile Responsive Layout:
- Add useMediaQuery, useIsMobile, useIsTablet, useIsDesktop hooks for device detection
- Create MobileHeader component with hamburger menu and action drawer
- Create BottomToolbar for mobile navigation (Files, Members)
- Create SlidePanel component for full-screen mobile sidebars
- Update RoomDetail.tsx with mobile/desktop conditional rendering
- Update RoomList.tsx with single-column grid and touch-friendly buttons
- Add CSS custom properties for safe areas and touch targets (min 44px)
- Add mobile viewport meta tags for notched devices

Open Room Access:
- All authenticated users can view all rooms (not just their own)
- Users can join active rooms they're not members of
- Add is_member field to room responses
- Update room list API to return all rooms by default

Admin Room Management:
- Add permanent delete functionality for system admins
- Add delete confirmation dialog with room title verification
- Broadcast room deletion via WebSocket to connected users
- Add users search API for adding members

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/modules/auth/users_router.py

@@ -0,0 +1,52 @@
+"""User management API endpoints
+
+Provides:
+- GET /api/users/search - Search users by name or email
+"""
+from fastapi import APIRouter, Depends, HTTPException, status, Query
+from sqlalchemy.orm import Session
+from typing import List
+from pydantic import BaseModel
+
+from app.core.database import get_db
+from app.modules.auth import get_current_user
+from app.modules.auth.services.user_service import search_users
+
+router = APIRouter(prefix="/api/users", tags=["Users"])
+
+
+class UserSearchResult(BaseModel):
+    """User search result"""
+    user_id: str
+    display_name: str
+
+    class Config:
+        from_attributes = True
+
+
+@router.get("/search", response_model=List[UserSearchResult])
+async def search_users_endpoint(
+    q: str = Query(..., min_length=1, description="Search query (name or email)"),
+    db: Session = Depends(get_db),
+    current_user: dict = Depends(get_current_user)
+):
+    """Search users by display_name or email
+
+    Returns up to 20 matching users. Requires authentication.
+    Search is case-insensitive and matches partial strings.
+    """
+    if not q or len(q.strip()) == 0:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail="Search query required"
+        )
+
+    users = search_users(db, q.strip(), limit=20)
+
+    return [
+        UserSearchResult(
+            user_id=user.user_id,
+            display_name=user.display_name
+        )
+        for user in users
+    ]